The present invention relates to voltage reference and regulator circuits, and more particularly, to a circuit which actively simulates the performance of a zener diode and which is capable of regulating currents at very low levels or providing very low reference voltages.
Zener diodes are extensively utilized in simple voltage regulator circuits, reference voltage circuits, overvoltage protection circuits, signal clipping circuits, etc. A zener diode is a specially treated PN junction with a relatively low reverse breakdown voltage, for example less than five volts. When a source of voltage is applied to the zener diode in the reverse direction (anode negative), a reverse current develops. As the reverse voltage is increased, eventually a so-called "avalanche breakdown" condition occurs, resulting in a large increase in current for a small change in voltage. A zener diode presents the effect of a very low impedance and therefore maintains a fairly constant voltage over a large range of currents. The voltage at which the avalanche breakdown occurs is referred to as the zener voltage. Thus, the current-voltage curve of a zener diode exhibits a sharp knee at the zener voltage. Since the resistance of the zener diode drops drastically at this point, it is necessary to limit the current flow with an external resistor. Avalanche breakdown of an operating zener diode is not destructive as long as the rated power dissipation of the junction is not exceeded.
Physical limitations in the fabrication processes make it difficult to fabricate a zener diode to regulate on currents less than approximately one milliamp, or to maintain reference voltages less than approximately three volts. It would be desirable to provide a circuit for actively simulating the performance of a zener diode at low current levels and low voltage levels so that regulating and reference functions could be performed at these low levels.